Matt Asner, executive director of Autism Speaks in Southern California, believes the actual numbers may be much higher.

"We believe that the number is 2 to about 2.5 percent of the population," said Asner.

The cause or causes of autism are still not known. Without any blood test or other medical tests for autism, diagnosis is not an exact science. It's identified by making judgments about a child's behavior. Symptoms of being on the autism spectrum can mean anything from being obsessed with certain items, rocking, not speaking, to simply looking away when talking to someone.

Experts don't know for sure what is driving the increase. One possibility: more awareness and access to screening. Dr. Carolina Pena-Ricardo with Children's Hospital Los Angeles says any child on the autism spectrum can benefit from early intervention.

"We know that children that are not that severely affected need the services. If the child has difficulty with socialization, they need the services to be able to be productive in society," said Pena-Ricardo.

Thursday's report is considered the most comprehensive on the frequency of autism. Researchers gathered data in 2010 from areas in 11 states - Alabama, Arizona, Arkansas, Colorado, Georgia, Maryland, Missouri, New Jersey, North Carolina, Utah and Wisconsin.

The report focused on 8-year-olds because most autism is diagnosed by that age. The researchers checked health and school records to see which children met the criteria for autism, even if they hadn't been formally diagnosed. Then, the researchers calculated how common autism was in each place and overall.

According to the report, autism is five times more common in boys. The CDC estimates 1 in 42 boys have autism, compared to 1 in 189 girls.

The study also noted a racial discrepancy. Researchers say white children are more likely to receive an early autism diagnosis than Latino or black children. While study authors don't explain the reasons why, many experts believe it has a lot to do with the lack of access to care in under-served communities.

"There needs to be more reach out to these communities," said Pena-Ricardo.

One change CDC officials had hoped to see, but didn't, was a drop in the age of diagnosis. Experts say a diagnosis can now be made at age 2 or even earlier. But the new report said the majority of children continue to be diagnosed after they turn 4.